Could he do it in 2012? If not he could do it in 2013, particularly in the first half of the year.
I am particularly interested if it was possible in 2012
It is very possible in 2012 (I'm pretty sure I got this right):
He is the clear favorite in Paris by a good amount IMO, He currently has 5390 points in the year-end race. Nadal "only" has 6795 points, this value will not change as he isn't playing the rest of the year. So Ferrer can end the year ranked #4 if:
He wins in Paris, having 6030 points going into London. Then he wins at least 2 RR matches and the semifinal. Giving him +800 points. This would put him at 6830, 35 points ahead of Nadal.
Or
If he only makes the final in Paris, he would have 5630 points going into the WTF, meaning he would have to win the WTF with at least 2 RR wins. This would give him +1300 points, or a 6930 total.
Or
If he loses to Llodra in the semis, he will have 5390 points, so if he wins the WTF without dropping a RR match, he will have 6890 points, just enough to edge out Nadal for the YE#4.
So there is a chance, but a very low one.